The best explanation for the role of external forces in the fall of the Ming dynasty is:
C. the expense of fighting external forces weakened the empire as new groups, such as the Jurchen, gained control.
The Ming dynasty faced significant external pressures, including invasions and incursions from various groups, which strained their military and economic resources. This weakening of the empire ultimately contributed to its inability to fend off internal rebellions and external threats, leading to its downfall.